1. Field of the Invention
This invention relates to a carriage apparatus having a carriage guided by guide means and movable in one direction.
2. Description of the Prior Art
A carriage apparatus having a carriage driven by drive means such as a feed screw or the like, guided by guide means provided on a base bed and movable in one direction has often been used in a machine tool or a measuring instrument.
A slide guide or an antifriction guide has generally been used as the guide means of such a carriage apparatus, but in a carriage apparatus using the conventional slide guide, the accuracy and lubricating condition of the guide surface relative to the movement of the carriage are not always uniform and on the other hand, in a carriage apparatus using an antifriction guide, fluctuation often exists in the accuracy of the guide surface relative to the movement of the carriage and the dimensional accuracy of the rolling members used and, therefore, in either of these carriage apparatuses, it has been difficult to obtain an accurate operating characteristic of the carriage.
As the guide means which has particularly taken the operating characteristic of the carriage into consideration, use has also been made of static pressure guide in which fluid having a pressure is supplied between guide surfaces effecting relative movement, and the carriage is supported and guided by the static pressure of this fluid. The fluid may be liquid or gas and generally, the liquid may be oil and the gas may be air.
Such static pressure guides support the carriage by a static pressure with respect to guide surfaces having a certain area and, therefore, an average effect is obtained with regard to the accuracy of the guide surfaces. Further, the resistance to the movement of the carriage is only the frictional resistance of the fluid, and thus the sliding resistance of the carriage is small, and such static pressure guides have excellent merits for use with a carriage which requires an accurate operating characteristic. Such static pressure guides have a further merit that the support rigidity of the carriage can be controlled to a desired value by controlling the pressure or flow rate of the fluid supplied between the guide surfaces effecting relative movement.
Static pressure guides using oil as the fluid are usually adapted to receive a supply of pressurized oil from a hydraulic device provided near the carriage apparatus, but it has a disadvantage that thermal displacement arises in the carriage apparatus because the oil pressurized by the hydraulic device generally has a temperature rise which imparts a variation to the temperature of the carriage apparatus. To prevent such thermal displacement of the carriage apparatus, it is necessary to eliminate the difference between the temperature of the oil supplied and the temperature of the carriage apparatus. To make these temperatures identical, however an oil temperature adjusting apparatus having an expensive cooling device or the like is often required. Also, the hydraulic device itself for pressurizing the oil is a heat source which affects the room temperature and thus, it is difficult to make the temperature of the carriage uniform with that of the supplied oil.
On the other hand, static pressure guides using air as the fluid, unlike the static pressure guides using oil, generally adopt a concentrated supply system in which a compressed air generation source is provided at a remote location. The pressurized air can be readily obtained at normal temperatures, and, therefore, such devices hardly suffer from the problem of the carriage being thermally displaced by the temperature of the air used. Also, air is very low in viscosity as compared with oil and this leads to the advantage that the resistance to the movement of the carriage and the generation of heat resulting from viscous friction caused by the movement of the carriage are negligibly small. However, the support rigidity and load capacity of the carriage are generally small, and this leads to the disadvantage that self-excited vibration which is said to be attributable to the response delay which in turn results from the fact that air is a compressible fluid is liable to occur.
For the purpose of improving the load capacity of the static pressure guides using air for for the purpose of improving the rigidity thereof, it has been proposed to provide pockets in the guide surfaces of the guides or to use a control throttle for controlling the gas supplied, but such a static pressure gas guide bearing particularly suffers from a disadvantage that self-excited vibration is liable to occur, thus making it difficult to provide a static pressure gas bearing having a great load capacity and rigidity for practical use.
A feed screw may suitably be used as the means for moving the carriage. Such feed screws typically have an axis parallel to the direction of movement of the carriage, and are supported on the base bed for rotation about the axis but against movement in the axial direction. A nut is threadably fitted on this feed screw so that displacement of the nut in the axial direction corresponding to rotation of the feed screw is transmitted to the carriage, whereby predetermined rectilinear movement of the carriage may be obtained. As a conventional carriage apparatus, one in which the nut is directly secured to the carriage is known.
In this carriage apparatus wherein the nut is secured to the carriage, the nut moves along the guide surface of the base bed for guiding the carriage and, in order that the feed screw may maintain a proper fitting condition with respect to the nut, the axis of movement of the nut must be properly coincident with the axis of the feed screw. However, in the case of a carriage apparatus in which very high accuracy of the carriage is required, it is difficult both from the viewpoint of machining accuracy of parts and the viewpoint of assembly accuracy to make the two axes exactly coincident with each other.
That is, in the conventional carriage apparatus wherein the nut is secured to the carriage, even if the guide surface for guiding the carriage is precisely machined and a precise feed screw is employed, errors inevitably exist in the degree of straightness of the guide surface, the degree of parallelism of the guide surface to the feed screw, the eccentricity of the feed screw or the mounting of the feed screw and nut. Therefore, if a precise fit eliminating a back-lash is adopted in the guide surface portion for guiding the carriage or in the threadable fitting portion between the nut and the feed screw, defective threadable fitting results from the deviation between the axes of the nut and feed screw and rotational torque of the feed screw fluctuates and it becomes impossible to achieve highly accurate operation of the carriage.
To overcome such disadvantages of the conventional carriage apparatus, there has been proposed a carriage apparatus in which the nut is not directly secured to the carriage but supported on the carriage through a plate spring having a flat surface parallel to the direction of the axis of the feed screw. In this carriage apparatus, the threadable fitting condition between the nut and the feed screw does not become extremely deflective even if a deviation is created between the axis of the nut moved by rotation of the feed screw and the axis of the feed screw, because the nut can be minutely displaced in a direction orthogonal to the axis of the feed screw by the plate spring. However, since the nut is supported on the carriage through the plate spring (resilient member), the restitutional force of the plate spring corresponding to the displacement of the nut acts on the nut and carriage and this leads to a disadvantage that inaccurate operation such as, for example, rolling, pitching or yawing is readily imparted to the carriage. Also, where the displacement of the nut is not parallel to the axis of the feed screw but is one which involves rotation with respect to an axis other than the axis of the feed screw, torsion is created in the plate spring and in such case, the restitutional force of the plate spring acting on the nut and carriage is considerably great.
Also, in the carriage apparatus wherein the nut is secured to the carriage through a plate spring, the carriage and nut are coupled together through a plate spring of relatively small rigidity and the plate spring itself has little or no attenuation property and this has led to a disadvantage that the carriage is liable to be vibrated by extraneous force acting on the carriage or by the vibration of a motor or the like for driving the feed screw.
The static pressure guide may be provided between a block made integral with the carriage and the feed screw, and such guides may include a static pressure gas bearing in which pressurized gas is supplied between a reception surface provided on a shaft and the bearing surface of a housing to form a thin film of pressurized gas between the two, and the shaft is supported on the housing by the static pressure produced in this thin film of pressurized gas. In this bearing, the shaft is supported without contacting the housing and the resistance to the movement of the shaft is only the shearing resistance of the gas, and the resistance to the movement of the shaft is very small and, therefore, this bearing is usually used in the rotating portion or the high-speed rotating spindle of a precise measuring instrument which requires a low torque. Also, the static pressure gas bearing usually uses compressed air as the gas, and such bearing using compressed air is also suited for the purpose of cleaning the apparatus. Thus, the static pressure gas bearing has excellent merits for the purposes of low torque, high-speed rotation and cleaning, but generally it has a disadvantage that it is low in load capacity and support rigidity. Another disadvantage of the static pressure gas bearing is that self-excited vibration called air hammer is liable to occur.
For the purpose of improving the load capacity or support rigidity of the static pressure gas bearing, it has also been proposed to provide bearing pockets in the bearing surface or to control the pressure or flow rate of the supplied gas by a control throttle. With these means, however, self-excited vibration is liable to occur particularly where the supplied fluid of the static pressure bearing is gas, and it is difficult to improve the performance of such static pressure gas bearing greatly.
A static pressure gas bearing device having overcome such disadvantages is described in Japanese Pat. No. 736914 issued to the same assignee as this invention. This is a composite bearing (static pressure gas bearing device) in which the gap between a bearing surface and a surface opposed thereto communicates with a supply source of liquid having a required viscosity to form a thin film of liquid in this gap, and a gas bearing is caused to bear a load and static rigidity while, on the other hand, generation of self-excited vibration of the gas bearing is restrained by the thin film of liquid.
This composite bearing is such that liquid is supplied to the gap between a shaft and the gas bearing to thereby form a thin film of liquid between the two and self-excited vibration of the shaft is restrained by the squeeze damping effect of the thin film of liquid, but it has suffered from a disadvantage that a liquid supply device is required for supplying liquid to the gap between the shaft and the gas bearing. Also, in a static pressure gas bearing which requires precision, it has also been necessary to pay attention to the temperature of the liquid supplied.